HALO FALL OF REACH:
The beginning of the Halo series. This starts the spartan program and the attack of the covenant destroying reach and and trying to annihilate the human race.
HALO THE FLOOD:
The second book to release. This book starts to talk mostly about the flood and the struggle to fight with spartans to marines.
HALO FIRST STRIKE:
The book pretty much shows the noble team from reach still this book released before REACH but shows how the noble was the last defense for planet Reach.
HALO GHOSTS OF ONYX:
This book is mostly about the spartan three program and how the young soldiers go out on an exploration mission in forerunner ruins go missing and it is about their survival to and the deaths of the spartans.